COURSE of the MONTH

Totaling 3 fields on form.

I've got 3 numeric/editable fields (Fld1,Fld2,Fld3) on a form, which are totaled into a fourth field (TotFld).
I want to have the total (TotFld) calculated whenever a change is made to any value in fields Fld1, Fld2, or Fld3.

I'd like to avoid using the 'Form Refresh' property. Possibly performing the calculation in the "Exiting" event of each field???

Can someone provide the Lotuscript and where it needs to be placed to get this to happen.